Molecular characterization of core lipids from halophilic archaea grown under different salinity conditions

Halorhabdus utahensis, Natronomonas pharaonis, Haloferax sulfurifontis and Halobaculum gomorrense were grown at salinity values between 10% and 30% NaCl (w/v). The strains represent four haloarchaeal genera and have a range of salinity optima. Analysis of core membrane lipids of each strain using gas chromatography–mass spectrometry (GC–MS) revealed structures consistent with saturated, unsaturated and polyunsaturated dialkyl glycerol diethers (DGDs) including both phytanyl (C₂₀) and sesterpanyl (C₂₅) isoprenoid chains. In addition, we observed three trends related to salinity: (i) the proportion of unsaturated DGDs increased with increasing NaCl concentration in the medium, (ii) strains with a higher optimal NaCl concentration had a higher proportion of unsaturated DGDs and (iii) C_25–20 DGDs occurred in the two strains with higher salinity optima, N. pharaonis and H. utahensis. The strong linear correlation between optimal growth salinity and fraction of unsaturated DGDs suggests that membrane lipid unsaturation is an important adaptation to specific salinity niches in archaeal halophiles. In addition, in three of the four strains, the fraction of unsaturated DGDs increased above a salinity threshold or in response to increasing salinity in the medium. Thus, halophilic archaea regulate membrane lipid unsaturation in response to environmental salinity change, regardless of their salinity optima.